Production of platinum group metal layer on a refractory

ABSTRACT

There is provided a process, and the product resulting therefrom, for coating a refractory block or substrate with a small particle size form of at least one platinum group metal whereby a fused layer of such metal is formed on the refractory and is directly and strongly adhered on a surface thereof. This technique can likewise be employed in such a manner that the layer is a bonding agent for purposes of bonding a separate sheet of a platinum group metal to the refractory which structure can then be employed as a glass contacting member in apparatus for handling molten glass.

This is a division, of application Ser. No. 862996, filed Dec. 21, 1977,now abandoned.

THE INVENTION

The present invention relates to methods for forming laminatedstructures and even yet more particularly relates to a method forforming a thin, fused layer consisting essentially of at least oneplatinum group metal on a refractory substrate. According to a furtherfeature of the present invention, the method can be employed to producea glass contacting member which can be employed in glass handlingapparatus.

There are many instances in the glass manufacturing industry whereinrefractory structures are employed which carry an outwardly disposedlayer or sheet consisting essentially of at least one platinum groupmetal. As exemplified in U.S. Pat. No. 2,777,254, which is herebyincorporated by reference, such articles can be employed as the feeder,or bushing, for producing fibrous glass products or they may be employedas a glass contacting part of the wall structure of a glass meltingtank. In addition to such uses, such structures may be employed asskimmer blocks in glass furnaces and they may likewise be employed aselectrodes in electric melt furnaces. Additionally, in many instances itis required, for example for quality control purposes, to monitor theelectrical conductivity of refractory structures. In the latterinstance, of course, the refractory must be provided with opposedelectrodes to allow such measurements.

According to the present invention, applicant provides for a simplereliable method for forming a thin, fused layer consisting essentiallyof at least one platinum group metal directly and strongly adhered onthe surface of a refractory substrate. This method may be used toproduce articles which may, in turn, be employed for the previouslyindicated purposes; when, for example, a thicker more durable layer ofthe platinum group metal is desired the method may be employed to bond aplatinum group sheet metal to the refractory structure.

Thus, in accordance with this invention, a process is provided forforming a thin fused layer consisting essentially of at least oneplatinum group metal directly and strongly adhered on the surface of arefractory substrate which comprises: depositing a solid, small particlesize form of the metal in a thin layer on the refractory substrate andheating the layer at a temperature and for a time sufficient to fuse thedeposited layer into a thin, fused, uniform layer; the solid smallparticle size form of the metal is produced by a process which comprisessubstantially quantitatively reducing a complex of the metal in anaqueous solution and precipitating the elemental form of the metaltherefrom as a solid small particle size form of the metal and thendrying the precipitated metal under conditions of temperature and timeso as to avoid sintering the particles. Suitably, the complex will be ahalide, for example chloride, or nitrite complex. Quite outstandingresults will be obtained when the metal is an alloy of platinum andrhodium wherein the platinum desirably will be in a major (greater than50% by weight) amount and the rhodium will be present in a minor amount.Outstanding results will be obtained by using an alloy consistingessentially of about 70 to 80 weight percent platinum and about 20 to 30percent rhodium. Desirably, the reducing and precipitating step willcomprise adding hydrazine or a hydrazine hydrate to an aqueous solutionof nitrate or halide complexes of platinum and rhodium and precipitatingplatinum and rhodium under alkaline conditions.

According to a further feature of this invention, prior to heating thedeposited layer to fusion, a sheet consisting essentially of a platinumgroup metal or an alloy of platinum group metals is positioned on theparticulate mass and the heating is effected so as to bond the sheet tothe refractory. Such a structure may be employed as a glass contacingmember in conventional glass handling apparatus such as alluded toabove.

Desirably, the small particle size platinum group material will be apurified alloy of platinum and rhodium which is a product of reclamationand is produced by a process which comprises: (a) dissolving aplatinum-rhodium alloy which is contaminated with metals or metalliccompounds in aqua regia; (b) adding sodium chloride to the solution inan amount of between about 70%-100% of the stoichiometric amountrequired to form disodium chloroplatinate and trisodium chlororhodate;(c) evaporating the solution of step b for a time sufficient to destroynitric acid and until hydrogen chloride fumes are substantially nolonger detectable; (d) contacting an aqueous solution of the product ofstep c with a sodium form cation exchange resin to separate contaminatemetal cations and provide an effluent containing reducible platinum andrhodium anionic complexes in an aqueous solution (e) reducing thecomplexes to elemental platinum and rhodium with a strong reducing agentand precipitating the small particle size alloy. Advantageously, priorto step e residual contaminates are removed by precipitating theresiduals under alkaline conditions and separating them by filtration.If ultra-high purity material is desired, subsequent to step "e" andprior to drying of the small particle size form alloy, contaminates arefurther removed from the precipitate by treating the precipitate with anaqueous solution containing effective sequestering amounts of asequestering agent and separating the treated precipitate. Quitesuitable results will be obtained when employing ethylenediaminetetraacetic acid or its alkali, for example sodium, metal salts.

The small particle size platinum group metals which are employed hereinare not, per se, new. In fact, they have been produced for many yearsbut to applicant's knowledge, no one has recognized the beneficialattributes of such small particle size platinum group metals or alloys.As is well known in the art, bushings, or glass feeders, employed in theproduction of fibrous glass products are manufactured from sheets of atleast one platinum group metal and, more typically, alloys thereof. Inthe process of manufacturing such bushings, the metals and alloysvolatilize and condense on adjacent refractory structures. Thesecondensed forms are then collected and initially separated from bulkresidual refractory by conventional gravimetric separation techniques toproduce a platinum group metal or alloy which is still somewhatcontaminated with other metals or metal compounds. The contaminated formof the alloy is then treated by the process indicated above to produce apurified small particle size form of the metal or alloy. Notwithstandingthe availability and production of such small particle size material,that material was sintered into masses and melted and then processedinto sheets to form bushings. That is, to the best of applicant'sknowledge, no one recognized that this material could advantageously beemployed in a process for forming a thin, fused layer consistingessentially of at least one platinum group metal onto a refractorysubstrate. For further particulars on the method for manufacturing thesmall particle size material contemplated for use herein, referenece maybe made to "Refining of Platinum and Rhodium by an Ion Exchange Process"appearing in "Industrial and Engineering Chemistry," April, 1956, page711 which is hereby incorporated by reference.

The refractories upon which a thin fused layer consisting essentially ofat least one platinum group metal, i.e., Pt, Ru, Rh, Pd, Os, Ir oralloys thereof are formed of any of the refractories conventionallyemployed in the glass manufacturing industry. Such refractories aregenerally discussed in "Handbook of Glass Manufacture", Volume 1,Section 7, (1974) by Tooley which is hereby incorporated by reference.Preferably, the refractory will have surface porosity, or will have arough surface so as to allow penetration of the particles into thesurface voids. Outstanding results will be obtained using a chromicoxide refractory.

The solid, small particle size form of the platinum group metal or alloythereof may be deposited onto the refractory substrate in anyconventional manner. For example, the material may be applied as a drypowder, as for example, by brushing a layer onto the refractorysubstrate or employing a doctor blade technique. If desired, theseparticles may be slurried in a suitable volatile carrier for example,alkanols, ketones, esters, ethers, or the mono and di-alkyl ethers ofethylene glycol, and their derivatives, in which case the slurry may,for example, be applied by flow coating, spraying and the like.

Generally, the small particle form material will be applied in athickness sufficient that upon fusing the resulting thickness will be inthe range of about 0.0005 to about 0.125 inches and preferably, at leastfor bonding, about 0.0005 to about 0.001. In the embodiment wherein thesolid small particle size material is to be employed to bond a sheet ofa platinum group metal to the refractory, or a sheet of an alloy of aplatinum group metal, such sheet will generally be on the order of atleast about 0.003 inch in thickness. Various heating cycles may beemployed to effect the fusion and/or bonding but generally the intensityof such cycles are, surprisingly, quite low. For example, the fusionand/or bonding may be effected without any significant pressure, i.e. atatmospheric pressure, at temperatures in the range of about 2000° F. or2200° F. to about 2700° F. The time for heating of course, will varywith the temperature but even at temperatures between 2200° F. and 2400°F. times on the order of about 1/2 to about 1 hour will be quitesatisfactory. No particular care or any special equipment is requiredfor the fusing and/or bonding step but, of course, in those instanceswhere a volatile carrier is employed that carrier will have to first beevaporated with suitable precautions taken depending on the nature ofthe material employed.

Thus, as indicated above, the process is quite simple and reliable and,as will be appreciated, can be practiced quite economically. Thus, whileevaporative coating techniques are known, for example, as disclosed inU.S. Pat. No. 2,799,600, it will be appreciated that such sophisticatedequipment need no longer be employed and coatings can now be producedeven in small thicknesses quite conveniently and economically.

While the foregoing sets forth the present invention with sufficientparticularity to enable those skilled in the art to make and use same,nonetheless there follows further exemplification of the presentinvention.

EXAMPLE 1

The solid form small particulate material which was employed was analloy consisting essentially of about 76-77% by weight platinum andabout 23-24% by weight rhodium. This alloy was a purified, reclaimedmaterial produced in accordance with the teachings of the aboveincorporated article appearing in "Industrial and EngineeringChemistry". The alloy was initially that which condensed on adjacentrefractory walls in the bushing manufacturing process and this alloy wasintially separated by conventional gravimetric techniques from the bulkof contaminate refractories and that separated alloy, which was stillcontaminated with metals and metal compounds, was dissolved in aquaregia. A sufficient quantity of solid sodium chloride is added to thesolution in an amount of about 70 mole percent of the stoichiometricequivalent required to form disodium chloroplatinate and trisodiumchlororhodate. This solution in turn is evaporated to dryness and 3 mlof hydrochloric acid per gram of platinum-rhodium alloy are added. Thatsolution is again evaporated to dryness and the hydrochloric acidtreatment is repeated twice with evaporations to destroy the nitricacid. The final evaporation is continued until hydrogen chloride fumesare substantially no longer detectable. The resulting salts are dilutedto a concentration of about 25 grams per liter and are heated until allhave dissolved. That solution is then pumped through a ion exchangecolumn containing 50 to 100 mesh sodium-form Dowex 50X8 cation exchangeresin. The resulting effluent, which contains about 10 grams of theplatinum group metals per 400 ml of effluent is then treated by heatingabout 2.5 liters of the solution containing the platinum and rhodium asanionic complexes to about 65° C. followed by the slow addition theretoof 40 ml of 85% hydrazine hydrate; sodium hydroxide is then added tomake the solution distinctly alkaline at which time the platinum groupmetals are coagulated as a solid, small particle size material. Thatalloy material may then be filtered and dried and employed ascontemplated herein. In a preferred practice however, the supernatantliquid from the substantially quantitative reduction is decanted and thesmall particle size alloy is washed twice, followed by decantation, withdistilled water. The small particle size alloy is then heated for aboutan hour at 85° C. in a slightly acidic solution of sodiumethylenediamine tetraacetate. The alloy is then washed twice, followedby decantation, and the sequestration is repeated in a solution madeslightly alkaline with sodium hydroxide. The small particle sizematerial is then filtered under vacuum and washed with water until thewater is substantially chloride-free. Finally, the small particle sizealloy is washed with methanol to remove excess water and then air-driedto produce a dry, solid, small particulate size form consistingessentially of platinum and rhodium.

Onto a porous refractory block (for example, a porous chromic oxide suchas that manufactured by Corhart Refractories Company under theirdesignation C-1215) the dry solid small particulate alloy is depositedand brushed into a thin layer on the surface thereof. The refractorywith its particulate coating is then heated at a temperature of about2400° F. for about 30 minutes and results in the formation of a stronglyadhered, uniform, fused coating consisting essentially of the platinumand rhodium alloy thereon. The thickness of the resulting coating was onthe order of about 0.004 inches and it is believed that part of theoutstanding adhesion was the result of the material flowing, at leastpartially, into some of the surface pores of the block.

The following illustrates an embodiment wherein electrodes are bonded toopposite sides of the refractory substrate for purposes of makingelectrical conductivity measurements. It will be apparent however thatonly one side of the substrate need be so treated and such products canbe employed as bushing materials and/or skimmer blocks and/or wallcomponents of a melting furnace. Onto a sheet (which had been cleaned bywashing with methanol or ethanol) of a platinum group metal (alloy ofabout 76-77% platinum and 23-24% rhodium) a small layer of the aboveproduced powder is brushed into a substantially uniform layer. Onto thatpowder layer there is then positioned the refractory substrate indicatedabove and on the exposed surface of the refractory substrate there isthen brushed another layer of the dry solid particulate form alloy. Ontothat outer layer there is then deposited another sheet of the type usedon the opposite side and the entire laminated structure is then heatedat a temperature of about 2400° F. for about 30 minutes. The result is alaminated structure in which the externally disposed sheets are stronglybonded to the refractory allowing reliable and accurate conductivitymeasurements.

While the foregoing describes the present invention, it will of coursebe apparent that modifications are possible which pursuant to the patentstatutes and laws do not depart from the spirit and scope thereof.

I claim:
 1. A process comprising heating an article comprised of a particulate layer of a small particle size alloy composition consisting essentially of platinum and rhodium disposed intermediate a refractory substrate and a sheet of platinum group metal or an alloy thereof so as to bond said sheet to said substrate, said small particle size alloy composition being produced by a process which comprises substantially quantitatively reducing platinum and rhodium complexes in an aqueous solution and precipitating the elemental form of said alloy composition therefrom as a solid, small particle size form thereof and drying said precipitate without sinterization.
 2. The process of claim 1 wherein said sheet is a sheet of a platinum-rhodium alloy. 